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<title>Thread safety guarantees</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="intrusive.thread_safety"></a><a class="link" href="thread_safety.html" title="Thread safety guarantees">Thread safety guarantees</a>
</h2></div></div></div>
<p>
      Intrusive containers have thread safety guarantees similar to STL containers.
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Several threads having read or write access to different instances is safe
          as long as inserted objects are different.
        </li>
<li class="listitem">
          Concurrent read-only access to the same container is safe.
        </li>
</ul></div>
<p>
      Some Intrusive hooks (auto-unlink hooks, for example) modify containers without
      having a reference to them: this is considered a write access to the container.
    </p>
<p>
      Other functions, like checking if an object is already inserted in a container
      using the <code class="computeroutput"><span class="identifier">is_linked</span><span class="special">()</span></code>
      member of safe hooks, constitute read access on the container without having
      a reference to it, so no other thread should have write access (direct or indirect)
      to that container.
    </p>
<p>
      Since the same object can be inserted in several containers at the same time
      using different hooks, the thread safety of <span class="bold"><strong>Boost.Intrusive</strong></span>
      is related to the containers and also to the object whose lifetime is manually
      managed by the user.
    </p>
<p>
      As we can see, the analysis of the thread-safety of a program using <span class="bold"><strong>Boost.Intrusive</strong></span> is harder than with non-intrusive containers.
    </p>
<p>
      To analyze the thread safety, consider the following points:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          The auto-unlink hook's destructor and <code class="computeroutput"><span class="identifier">unlink</span><span class="special">()</span></code> functions modify the container indirectly.
        </li>
<li class="listitem">
          The safe mode and auto-unlink hooks' <code class="computeroutput"><span class="identifier">is_linked</span><span class="special">()</span></code> functions are a read access to the container.
        </li>
<li class="listitem">
          Inserting an object in containers that will be modified by different threads
          has no thread safety guarantee, although in most platforms it will be thread-safe
          without locking.
        </li>
</ul></div>
</div>
<div class="copyright-footer">Copyright © 2005 Olaf Krzikalla<br>Copyright © 2006-2015 Ion Gaztanaga<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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